(1) Field of the Invention
The present invention relates to devices and methods for coding image.
(2) Description of the Related Art
In resent years, with the increase of pixels in imaging devices used in image capturing apparatuses such as digital still cameras and digital camcorders, an amount of image data processed by an integrated circuit embedded in such an apparatus has been increased. In order to allow the integrated circuit in the image capturing apparatus to handle a large amount of image data, speeding-up of operation frequency, increase of a memory capacity, and the like are considered to secure a bus width of data transfer in the integrated circuit. However, the above results directly in cost increase.
Moreover, in general, after the integrated circuit completes all image processing, the image capturing apparatus such as a digital still camera or a digital camcorder performs compression processing for the image data when the image data is to be recorded onto an external recording device such as a memory card. After that, the image capturing apparatus records the compressed data generated by the compression processing onto the external recording device. In the compression processing, a coding method such as Joint Photographic Experts Group (JPEG) or Moving Picture Experts Group (MPEG) is used. Therefore, it is possible to record, onto the external recording device, image data with a size larger or with an amount larger than the case where the image data is not compressed.
Patent Reference 1 discloses a technique of performing such image data compression processing, not only for data for which image processing has been performed, but also for pixel signals (raw data) provided from an imaging device. This technique makes it possible to increase the number of continuous capturing pictures of the same size for the same memory capacity. In addition, Patent Reference 1 discloses a digital signal compression coding device and a digital signal decoding device, each of which does not need a memory to realize a high compression rate in a state where a low coding operation processing load is secured.
On the other hand, Patent Reference 2 discloses an image coding device capable of printing, at high accuracy and at high speed, a print object on which various pieces of image data having different properties are mixed, such as a poster or an advertisement on which single-color characters or graphics are included in a natural image such as a photograph. Since image properties of the characters or graphics differ from image properties of the natural image, adoption of the same coding method deteriorates image quality.
The image coding device disclosed in Patent Reference 2 includes an irreversible coding unit for coding pixel data by fixed length codes, a reversible coding unit, a surplus code counter, and a coding control unit.
When a compression rate of pixel data of a to-be-coded pixel is higher than a specified compression rate, the reversible coding unit codes the pixel data of the to-be-coded pixel by leaving surplus code(s) from fixed length codes. On the other hand, when a compression rate of pixel data of a to-be-coded pixel is lower than the specified compression rate, the reversible coding unit codes the pixel data of the to-be-coded pixel by consuming the surplus code(s) added to the fixed length codes.
The surplus code counter counts up the surplus code(s) from the fixed length codes when the reversible coding unit codes the image data, and counts down the surplus code(s) consumed by the reversible coding unit. The coding control unit performs processing for continuing coding operations of the reversible coding unit, in a range where a count value of a surplus counter is not lower than the specified compression rate. The image coding device performing the above-described processing can improve data reproducibility.
Prior Arts
Patent References
    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2007-036566    Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2007-181051